This invention relates to a control system and method based on determining whether a mixing system at an oil or gas well produces a mixture having an actual output characteristic that corresponds to a computed ideal output characteristic for the mixing system. In a particular aspect, such system and method determine the relationship between the actual sand concentration of a slurry mixed at an oil or gas well and an ideal sand concentration computed at the well using a mathematical model of the mixing system and actual sensed characteristics of the mixing system applied to the model.
Various types of fluids can be mixed at an oil or gas well. One type I will refer to for purposes of illustration is a fracturing fluid. A fracturing fluid includes at least a base fluid, typically including water, and a dry material, such as sand, referred to as proppant. Once mixed, the fracturing fluid is pumped into the well under pressure sufficient to fracture a subterranean formation and hold the fractures open with the proppant, thereby facilitating the production of hydrocarbons (oil or gas) found in the formation.
To make a fracturing fluid at a well, the base fluid is pumped into a mixing tub and the dry material is added. These inlet flows can be manually controlled in an attempt to obtain a mixture having some predesigned characteristic that can be measured, such as density or sand concentration derived from density. For example, an engineer might design a fracturing job wherein a fracturing fluid with a certain density is to be made and pumped down into the well. An operator at the well has the job plan and adjusts controls for the base fluid and/or proppant to try to obtain this density in the output mixture. The operator looks at the mixture and at an output densimeter and adjusts one or more of the inlet flows as needed. If the output characteristic is not being obtained by the operator's manual adjustments of the inlet flow(s), the operator knows that there is some mechanical problem in the mixing system.
Such mixing can also be done under automatic control implemented with a programmed computer. The computer is programmed with the desired characteristic (for example, the predesigned density in the example above) and it automatically controls one or more of the inlet flows in response to feedback from various sensors (for example, flow meters, level sensors and densimeters). Such automatic control is more efficient than manual control; however, if a desired output is not being obtained by such automatic control, it may not be readily apparent whether there is a problem with the controller or with the mechanical aspects of the mixing system. For example, that actual density is not within a given tolerance of the desired density at any one time is not necessarily indicative of whether the error is occurring because of a control system problem or because of a mechanical problem in the mixing process.
On the one hand, there is the need for an automatic control system to make the mixing at an oil or gas well more efficient as compared to manual control. This need has been met by the aforementioned type of automatic computer control that tries to produce a fracturing fluid having a density (for example) that equals a predetermined density set point entered in the computer. This, however, is still somewhat qualitative in that it in effect performs automatically only what the operator previously tried to do manually--that is, trying to match a measured output characteristic to a predetermined set point independent of the mixing system's capabilities.
On the other hand, there is the need for an independent or distinct control system and method that determines what the mixing system should be doing based on the actual inputs into the mixing system (which are caused such as by the prior primary automatic control) and what the mixing system is actually doing, regardless of the actual operating parameters under which the primary automatic control system is operating. This requires an analysis of the mixing system, not simply some predefined set point. Satisfying this need will facilitate determining whether a problem lies in the primary automatic control system or in the mechanical system. It is to this need that the present invention is directed.